a head which has at least a reading element, is provided with a heater which varies a protrusion value by thermal expansion which accompanies electric power distribution and heating, and accesses data by flying above a rotating recording medium;

an amplitude measurement unit which measures an amplitude of a read signal from a preamble unit in a servo area of a predetermined frame number m per one rotation of the recording medium by the reading element for a predetermined cycle number n while increasing an electric power amount distributed to the heater in the state in which the head is positioned to a track on the recording medium;

a mean amplitude calculating unit which calculates a mean amplitude at each of the heater electric power amount from an amplitude measurement value which is measured by the amplitude measurement unit and corresponding to a number which is the predetermined frame number m multiplied by the predetermined cycle number n;

a contact determination unit which calculates a contact determination value based on a variation amount in the heater electric power amount and a variation amount in the mean amplitude, and determines that the head is in contact with the recording medium when the contact determination value is lower than a predetermined threshold value; and

a flying height calculating unit which calculates a flying height of the head with respect to a surface of the recording medium from the heater electric power amount at the point when contact is determined by the contact determination unit and outputs the flying height; and

wherein

when the mean amplitude of a current time is μ(i), the mean amplitude of a previous time when the heater electric power is P(i−1) is μ(i−1), the heater electric power of the current time is P(i), and the heater electric power of the previous time is P(i−1),

the contact determination unit calculates the contact determination value as a contact determination value S1(i) which is a ratio of a variation amount Δμ(i) of the mean amplitudes with respect to a variation amount ΔP(i) of the heater electric power amounts of the previous time and the current time.